Chromatographic effluent, which may be liquid or gaseous in form, is routinely subjected to spectroscopic analysis, often mass spectrometry, in order to try and elucidate the eluting compounds contained in the effluent. For over three decades, computer control of mass spectrometer apparatus has enabled multiple mass spectra across a wide mass range to be acquired at a repetition rate that exceeds the elution period of any component of a complex mixture of a gas chromatographic effluent, for example, and this has allowed so-called mass chromatograms to be generated. Mass chromatograms are a record of the abundance of a mass measured from the effluent as a function of the elution or retention time, or scan number, as multiple mass spectral scans are performed during the elution period. Analysis of mass chromatograms enables the detection of eluted compounds which would otherwise be difficult or impossible to resolve, and can give information on the nature of the detected compounds.
Biller and Biemann (Anal. Lett., 7(7), 515-528, (1974)) described the analysis of mass chromatograms to produce so-called reconstructed mass spectra, which consist only of masses that have maximised in abundance at the same or closely matched elution times (the same or an immediately preceding scan number). This method produced simplified mass spectra at each scan number as all masses whose abundance was not maximising at or adjacent that scan number were rejected from the spectrum associated with that scan-increasing, steady and declining abundance mass signals were discarded from the spectrum leaving only those that maximise. This enabled less ambiguous identification of eluting compounds. As a further step, Biller and Biemann described combining all those masses which maximised in abundance at the same or closely matched elution times to create a so-called mass resolved gas chromatogram formed from the sum of the abundances of all such co-eluting mass peaks. The mass resolved gas chromatogram is similar in nature to the total ion chromatogram (TIC), but being composed only of masses whose abundance maximises at or close to the elution time, advantageously reveals previously hidden chromatographic structure, which allows eluting compounds to be more easily resolved graphically. However, in many situations, the resolution provided still may not be sufficient.
Dromey et.al. (Anal. Chem., 48(9), 1368-1375, (1976)) described accurately locating the elution time of components by analysis of mass chromatograms (fragmentograms, as termed by Dromey et. al.) which are only of masses unique to the eluting component relative to its neighbours—termed singlet peaks. The approach involved use of both the number of such singlets that maximise at a given elution time, and the total ion abundances above background at those maxima. They described their approach as looking for clusters of fragmentogram peaks. Analysis of the mass chromatograms included thresholding to remove background, in which a model peak profile was used, derived from the current data set. They extended their analysis to doublets and in theory to multiplets generally.
Colby (J. Am. Soc. Mass Spectrom., 3, 558-562, (1992)) described a so-called deconvoluted total ion current (DTIC) with a resolution of 0.1 scans using a three-point quadratic fit centroid calculation for peaks in the mass chromatograms, coupled with baseline correction (by an undisclosed method). Related masses were grouped on the basis of the retention time of the centroids, to a higher resolution than measurement interval.
Despite the advances provided by the prior art methods described above, there nevertheless remains a need to significantly improve the processing of chromatographic data, especially mass chromatographic data, e.g. to increase the ability to resolve and identify eluting compounds and/or reduce complexity of the processing. Against this background the present invention has been made.